By sharing a sequence of random bits, two users obtain with provable security the capacity to transmit messages that cannot be deciphered by an eavesdropper (one-time-pad protocol) as well as to distinguish legitimate messages from forged or altered ones (authentication procedures). Creation of fresh key information and the means to securely transmit them is a central goal in cryptography. Set aside security aspects, speed is another requirement of modern communication systems that excludes use of couriers to transport fresh keys. Communication systems only secured by mathematical complexities rests on unproven assumptions such as the difficulty of factoring large numbers and may vulnerable either by advances in computational power or new mathematical insights. Physical cryptography, on the other hand, can create schemes providing two users, at distinct locations, with on-demand copies of a secure sequence of random bits of arbitrary length and at fast rates without relying on mathematical complexities. These schemes could be of high value for commercial systems operating over long distances. Communication with perfect secrecy using the securely transmitted keys could be guaranteed over an insecure channel in Vernam's sense of a one-time-pad. Technology advances, therefore, such as enhanced computational power, should not affect the security of these schemes.
The BB84 quantum protocol for key distribution and the paradigm among protocols using single photons, has been used in short distance applications but not in long distance networks. See for example, C. H. Bennett and G. Brassard, “Quantum cryptography: public-key distribution and coin tossing”, Proc. IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, pp. 175-179, 1984; and N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Rev. Mod. Phys. 74 pp. 145-195 (2002). R. J. Hughes, J. E. Nordholt, G. L. Morgan and C. G. Peterson, QELS Conference, OSA Technical Digest, Vol. 74, p. 266 (2002). One fundamental reason is that the same no-cloning theorem that guarantees its security level prohibits the signal amplification necessary in long-haul communication links. No practical alternative quantum scheme using quantum repeaters or entangled states has yet been proposed although theoretical studies exist. Other practical impediments are the slow speed of the photon sources and the large recovery time of single photon detectors. The speed difficulties and the impossibility of amplification rule out the single photon protocols to be used in long haul commercial communication lines. However, one has to assume that key distribution systems could be obtained in a secure way within a few years through BB84 key distribution systems by using satellites. See for example, M. Aspelmeyer, T. Jennewein, and A. Zeilinger, M. Pfennigbauer and W. Leeb, quant-ph/0305105 v1, 19 May 2003. The expected rates of this quantum key distribution process are expected to be low and, therefore, a scheme for fast and secure key distribution that could use a starting shared secret key obtained by this or another slow secure method is expected to be of great utility for applications demanding overall speed and security. The cryptographic system object of this patent application is direct to this use.
“Quantum key distribution (QKD) using non-orthogonal macroscopic signals,” U.S. Pat. No. 5,515,438, uses non-orthogonal quantum states to distribute random information, suitable for use as a key for encryption and authentication, between two users who share secret information initially. It differs from previous QKD schemes in using macroscopic signals instead of single photons. This system is bound to two bases states to encrypt-decrypt the desired information. The limitation of two bases gives brute force attacks on the key a high probability of success even for coherent light states with mesoscopic number of photons. The quantum noise of light in this case would cover at most the two bases in the system and its protection relies mostly on the secret key used and the associated mathematical complexity. A system with protection mostly derived from the quantum noise of light is preferable.
A ciphering scheme utilizing an M-ry bases system that was implemented for data encryption has been proposed. See for example, H. P. Yuen, in “Ultra-secure and Ultra-efficient Quantum Cryptographic Schemes for Optical System, Networks and the Internet”, unpublished, DARPA/Northwestern University Project (2000); G. A. Barbosa, E. Corndorf, and P. Kumar, “Quantum Cryptography with Coherent-state Light: Demonstration of a Secure Data Encryption Scheme Operating at 100 kb/s”; Quantum Electronics and Laser Science Conference, OSA Technical Digest 74, pp. 189-190 (2002); G. A. Barbosa, E. Corndorf, P. Kumar, and H. P. Yuen, Phys. Rev. Lett. 90, 227901 (2003); and also in quant-ph/0212018 v2 21 Apr. 2003; G. A. Barbosa E. Corndorf, P. Kumar, H. P. Yuen, G. M. D'Ariano, M. G. A. Paris, and P. Perinotti, “Secure Communication using Coherent States”, in The Sixth Int. Conference on Quantum Communication, Measurement and Computing, July 2002 (Rinton Press, Princeton, April 2003), pp. 357-360; and E. Corndorf, G. A. Barbosa, C. Liang, H. P. Yuen, and P. Kumar, “High-speed data encryption over 25 km of fiber by two-mode coherent-state quantum cryptograph”, Optics Letters 28, 2040-2042 (2003). Basically, in these cryptographic prototypes, the quantum noise inherent to coherent states forces different measurement results for the eavesdropper and the legitimate users that use a shared key in their measurements. This noise will increase the observational uncertainty preponderantly for the eavesdropper, hereafter named Eve (E), rather than Alice (A) and Bob (B), hereafter named legitimate users. Although this noise is irreducible by nature to all observers, the knowledge of a key allows A and B to achieve a much higher resolution than the one obtained by Eve. The very simple idea behind this is that, for each bit, the noise is distributed without control among the output ports in Eve's measurement apparatus while A and B use the key to select a single output port where the noise does not practically affect bit readings. For other known systems see, “Fast and secure key distribution using mesoscopic coherent states of light”, G. A. Barbosa, Phys. Rev. A 68, 052307 (2003); arXiv:quant-ph/0212033 (2002).